The production of polymer articles, particularly filaments and ribbons, having very high tensile strength and modulus, from solutions of high molecular weight polymers, specifically high molecular weight linear polyethylene, has now been described in a number of patents. See, for instance, U.S. Pat. Nos. 4,344,908; 4,411,845; 4,422,993; 4,430,383; and 4,436,689. Also see U.S. Pat No. 4,137,394.
Generally speaking, in these known processes, a solution having a concentration of at most about 20 wt.% of a high molecular weight polymer is spun or extruded through an aperture, either round or slit-shaped, to form a filament or ribbon. This filament or ribbon can be subsequently converted to form a gel-state filament or ribbon by cooling to a temperature below the gel point. Thereafter, such gel filament or ribbon may be stretched or drawn at an elevated temperature, either with or without first removing all or part of the solvent.
It has also been recognized that in such techniques it is very important that the polymer solutions used be homogeneous; in the absence of a high level of homogeneity the quality of the filaments or ribbons, and the drawability or stretchability in the gel state, will be seriously adversely affected, and indeed will be erratic over the length of the formed material.
Typically, in the past, such solutions of the polymer have been prepared by stirring the high molecular weight polymer in a suitable solvent to form the solution. It has been recognized, however, that to form such solutions is not an easy task, and becomes rather time-consuming. This is partially due to the fact that when working with the very high molecular weight polymer materials involved (for instance, polyethylene having a weight average molecular weight of at least 4.times.10.sup.5, or especially above about 8.times.10.sup.5) the mechanical forces applied during the stirring operation tend to elongate the normally coiled polymer molecules. As a result of this effect a retractive force is created by which the molecule seeks to retract itself into a coil again. The ultimate observed result in the practice of the conventional stirring techniques is that the high molecular weight polymer molecules will tend to gather around the center of the stirring rod, or other device, and to climb up against the stirring rod or device itself. This has been described in the literature as the so-called Weissenberg effect.
This effect, which is indigenous to the ordinary stirring of these high molecular weight polymers thus creates a major problem in attempting to form homogeneous solutions thereof. The Weissenberg effect is enhanced as the solutions are more vigorously or rapidly stirred, and as the polymer concentration is increased and as the molecular weight of the polymer in increased. It will be understood that under conditions such that the Weissenberg effect manifests itself, non-homogeneous polymer solutions result, with deleterious effects on the properties of the gel spun filaments, ribbons, or films.
Effectively, homogeneous solutions can only be prepared by these techniques if very slow stirring rates are employed, and if relatively dilute solutions of the polymer are used. It will also be appreciated that as the solution concentration increases, the solutions take on extremely high viscosity characteristics. Under these circumstances, stirring becomes more arduous and moreover it becomes increasingly difficult thereafter to feed and deliver the highly viscous solutions to a spinning head or to a conventional extrusion device to form the gel filament or tape.
A further disadvantage is that as the time for stirring increases, which is required when slow stirring speeds must be employed, the difficulty occurs that degradation of the polymer molecules will result unless extreme precautions are taken to exclude the presence of oxygen.
Further, because of the corresponding effect of a viscosity increase the process also becomes increasingly difficult as the molecular weight of the plymer molecules increases.
These problems have variously been recognized in the prior art as is illustrated by U.S. Pat. No. 4,137,394 which effectively discusses the slow rate of crystallization and consequently the slow rate of stirring required, for that process. Further, in U.S. Pat. No. 4,413,110, there is described a process for suspending high molecular weight polyolefins in paraffinic oil, wherein it is indicated that the suspension must be slowly stirred at elevated temperatures for many hours. These are precisely the conditions under which degradation of the polymer, and also segregation of polymer molecules of differing molecular weights, may occur, absent special precautions, so that, again, homogeneous solutions are not readily obtained.
There are, accordingly, numerous disadvantages to the presently known procedures for preparing these high molecular weight solutions and with respect to their spinning or extrusion through an orifice or aperture. The problems are such that high costs are involved, particularly when the fundamental process is attempted to be applied on a large, commercial scale, and it also becomes extremely difficult to carry out the known processes in a continuous, as opposed to a batch, procedure.